174 THE OIL-BEARING LIMESTONE OF CHICAGO. [X. 



petroleum. The total produce of the great Pennsylvania oil- 

 region for the ten years from 1860 to 1870 is estimated at 

 twenty-eight millions of barrels of petroleum, or less than 

 would be contained in four square miles of the oil-bearing 

 limestone formation of Chicago. 



It is not here the place to insist upon the geological condi- 

 tions which favor the liberation of a portion of the oil from such 

 rocks, and its accumulation in fissures along certain anticlinal 

 lines in the broken and uplifted strata. These points in the 

 geological history of petroleum were shown by me in my first 

 publications on the subject in March and July, 1861, referred 

 to on the next page, and independently, about the same time, 

 by Professor E. B. Andrews in this Journal for July, 1861.* 



The proportion of petroleum in the rock of Chicago may be 

 exceptionally large, but the oleiferous character of great thick- 

 ness of rock in other regions is well established, and it will 

 be seen from the above calculations that a very small propor- 

 tion of the oil thus distributed would, when accumulated along 

 lines of uplift in the strata, be more than adequate to the sup- 

 ply of all the petroleum wells known in the regions where 

 these oil-bearing rocks are found. With such sources exist- 

 ing ready formed in the earth's crust, it seems to me, to say the 

 least, unphilosophical to search elsewhere for the origin of 

 petroleum, and to imagine it to be derived by some unex- 

 plained process from rocks which are destitute of the sub- 

 stance. 



* American Journal of Science (2), XXXII. 85. See also papers on the 

 subject by Andrews and by Professor Evans, Ibid. (2), XL. 33, 334 ; and one 

 by the author (2), XXXV. 170 ; also Report Geological Survey of Canada, 

 1866, pp. 256, 257. 



